Diamond crystals are useful in various industrial applications. For example, diamond grains may be used in surface polishing, in the manufacture of drill bits, and as conductive filler materials for polymers and elastomers. Liquid suspensions of diamond grains may be used for lubrication, thermal management, or grinding.
Cutting elements used in earth-boring tools often include polycrystalline diamond compact (often referred to as “PDC”) cutting elements, which are cutting elements that include cutting faces of a polycrystalline diamond material. Polycrystalline diamond material is material that includes inter-bonded grains or crystals of diamond material. In other words, polycrystalline diamond material includes direct, inter-granular bonds between the grains or crystals of diamond material. The terms “grain” and “crystal” are used synonymously and interchangeably herein.
Polycrystalline diamond compact cutting elements are formed by sintering and bonding together relatively small diamond grains under conditions of high temperature and high pressure in the presence of a catalyst (for example, cobalt, iron, nickel, or alloys or mixtures thereof) to form a layer or “table” of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as high-temperature/high-pressure (or “HTHP”) processes. The cutting element substrate may comprise a cermet material (i.e., a ceramic-metal composite material) such as cobalt-cemented tungsten carbide. In such instances, the cobalt or other catalyst material in the cutting element substrate may diffuse into the diamond grains during sintering and serve as the catalyst material for forming the inter-granular diamond-to-diamond bonds, and the resulting diamond table, from the diamond grains. In other methods, powdered catalyst material may be mixed with the diamond grains prior to sintering the grains together in an HTHP process. Methods of forming polycrystalline compacts with interstitial materials are described in U.S. Patent Application Publication No. 2011/0061942 A1, “Polycrystalline Compacts Having Material Disposed in Interstitial Spaces Therein, Cutting Elements and Earth-Boring Tools Including Such Compacts, and Methods of Forming Such Compacts,” published Mar. 17, 2011, the disclosure of which is incorporated herein in its entirety by this reference.
Upon formation of a diamond table using an HTHP process, catalyst material may remain in interstitial spaces between the grains of diamond in the resulting polycrystalline diamond table. The presence of the catalyst material in the diamond table may contribute to thermal damage in the diamond table when the cutting element is heated during use, due to friction at the contact point between the cutting element and the rock formation being cut.
PDC cutting elements in which the catalyst material remains in the diamond table are generally thermally stable up to a temperature of about seven hundred fifty degrees Celsius (750° C.), although internal stress within the cutting element may begin to develop at temperatures exceeding about four hundred degrees Celsius (400° C.) due to a phase change that occurs in cobalt at that temperature (a change from the “beta” phase to the “alpha” phase). Also beginning at about four hundred degrees Celsius (400° C.), there is an internal stress component that arises due to differences in the thermal expansion of the diamond grains and the catalyst at the grain boundaries. This difference in thermal expansion may result in relatively large tensile stresses at the interface between the diamond grains, and contributes to thermal degradation of the microstructure when PDC cutting elements are used in service. Differences in the thermal expansion between the diamond table and the cutting element substrate to which it is bonded may further exacerbate the stresses in the PDC cutting element. This differential in thermal expansion may result in relatively large compressive and/or tensile stresses at the interface between the diamond table and the substrate that eventually lead to the deterioration of the diamond table, cause the diamond table to delaminate from the substrate, or result in the general ineffectiveness of the cutting element.
Furthermore, at temperatures at or above about seven hundred fifty degrees Celsius (750° C.), some of the diamond crystals within the diamond table may react with the catalyst material causing the diamond crystals to undergo a chemical breakdown or conversion to another allotrope of carbon. For example, the diamond crystals may graphitize at the diamond crystal boundaries, which may substantially weaken the diamond table. Also, at extremely high temperatures, in addition to graphite, some of the diamond crystals may be converted to carbon monoxide and/or carbon dioxide.
In order to reduce the problems associated with differences in thermal expansion and chemical breakdown of the diamond crystals in polycrystalline diamond cutting elements, so-called “thermally stable” polycrystalline diamond compacts (which are also known as thermally stable products, or “TSPs”) have been developed. Such a thermally stable polycrystalline diamond compact may be formed by leaching the catalyst material (e.g., cobalt) out from interstitial spaces between the inter-bonded diamond crystals in the diamond table using, for example, an acid or combination of acids (e.g., aqua regia). A substantial amount of the catalyst material may be removed from the diamond table, or catalyst material may be removed from only a portion thereof. Thermally stable polycrystalline diamond compacts in which substantially all catalyst material has been leached out from the diamond table have been reported to be thermally stable up to temperatures of about twelve hundred degrees Celsius (1,200° C.). It has also been reported, however, that such fully leached diamond tables are relatively more brittle and vulnerable to shear, compressive, and tensile stresses than are non-leached diamond tables. In addition, it is difficult to secure a completely leached diamond table to a supporting substrate. In an effort to provide cutting elements having diamond tables that are more thermally stable relative to non-leached diamond tables, but that are also relatively less brittle and vulnerable to shear, compressive, and tensile stresses relative to fully leached diamond tables, cutting elements have been provided that include a diamond table in which the catalyst material has been leached from a portion or portions of the diamond table. For example, it is known to leach catalyst material from the cutting face, from the side of the diamond table, or both, to a desired depth within the diamond table, but without leaching all of the catalyst material out from the diamond table.